History of the Atom

Ancient Greek Contribution

• Democritus (c. 500 BC)
  • Proposed the idea of atomic theory.
  • Suggested everything is made of tiny, indivisible particles called atoms.
  • Atoms are separated by empty space.

19th Century Developments

• John Dalton (1800s)
  • Described atoms as solid spheres.
  • Proposed that different elements are made of different types of spheres.

Discovery of Electrons

• J.J. Thompson (1897)
  • Introduced the Plum Pudding Model.
  • Conducted experiments showing atoms are not solid spheres.
  • Discovered negatively charged particles (electrons).
  • Proposed atoms are positive spheres with embedded electrons.

Rutherford’s Nuclear Model

• Ernest Rutherford (1909)
  • Conducted the gold foil experiment using alpha particles.
  • Observed deflection of some particles, disproving the Plum Pudding Model.
  • Proposed the Nuclear Model:
    • Atoms have a compact nucleus with all the positive charge.
    • Electrons exist in a cloud around the nucleus.

Bohr’s Model

• Niels Bohr (1913)
  • Suggested electrons orbit the nucleus like planets around the sun.
  • Introduced the concept of electron shells.
  • Explained why atoms do not collapse.
  • Model supported by subsequent experiments and remains largely unchanged today.

Further Discoveries

• Ernest Rutherford
  • Identified the positive charge in the nucleus as protons.
• James Chadwick
  • Provided evidence for neutrons, neutral particles in the nucleus.

Conclusion

• The understanding of atomic structure has evolved significantly from Democritus to Chadwick.
• The modern atomic model is based on electrons orbiting a nucleus composed of protons and neutrons.